1. Field of the Invention
This invention relates to a process for the production of alkali dichromates and chromic acid by electrolysis of alkali monochromate or alkali dichromate solutions.
2. Description of Related Art
According to U.S. Pat. No. 3,305,463 and CA-A-No. 739,447, the electrolytic production of dichromates and chromic acid takes place in electrolysis cells of which the electrode compartments are separated by cation exchanger membranes. In the production of alkali dichromates, alkali monochromate solutions or suspensions are introduced into the anode compartment of the cell and are converted into an alkali dichromate solution by the selective transfer of alkali ions through the membrane into the cathode compartment. For the production of chromic acid, alkali dichromate or alkali monochromate solutions or a mixture of alkali dichromate and alkali monochromate solutions are introduced into the anode compartment and converted into solutions containing chromic acid. Sodium monochromate and/or sodium dichromate are generally used for these processes. In both processes, an alkaline solution containing alkali ions is obtained in the cathode compartment, consisting for example of an aqueous sodium hydroxide solution or, as described in CA-A-No. 739,447, of an aqueous solution containing sodium carbonate.
To produce alkali dichromate or chromic acid crystals, the solutions formed in the anode compartments of the cells are concentrated; the crystallization of sodium dichromate can be carried out, for example, at 80.degree. C. and the crystallization of chromic acid at 60.degree. to 100.degree. C. The products crystallized out are separated off, optionally washed and dried.
According to DE-A No. 3 020 260, anodes of lead and lead alloys are suitable as anode materials. These anode materials have the disadvantage that, as a result of corrosion, lead ions can enter the solution in the anode compartment, leading to contamination of the alkali dichromates and chromic acid produced. According to DE-A No. 3 020 260, other suitable anodes are so-called dimensionally stable anodes which consist of a valve metal, such as titanium or tantalum, with an electrocatalytically active layer of noble metal or a metal noble oxide. However, anodes of this type have only a limited useful life of less than 100 days, particularly at electrolysis temperatures above 60.degree. C. and anodic current densities of 2-5 kA/m.sup.2. Useful lives as short as these are inadequate for the economic production of alkali dichromate and chromic acid by electrolysis.